1. Field of the Invention
There is a growing need for a quantitative means to determine the magnetic force generated between a Magnetic Structure (a permanent magnet with pole conduits (properly sized high permeability ferromagnetic material) placed on the magnetic pole faces) and its Target (the ferromagnetic material to which the Magnetic Structure attaches). This need has not been adequately met due to deficiencies in current technology.
2. Prior Art
U.S. Pat. No. 4,237,455 issued to Beckley et al. (1980), Improvements in Safety Device for Lifting Magnets (“the '455 patent”), describes a magnetic field flux sensor combined with a load cell, to provide an output that is indicative of the safety margin between lifting tension and magnetic field available for lifting. The device, intended for lifting magnets, uses a “sensor means” to produce a signal representative of flux produced by a device in a magnetic circuit that includes a body to be lifted. In this design, the “sensor means” is depicted in the patent by a moveable pole piece or plunger that is moved downwardly into contact with a load through a sensing coil. The sensing coil responds to the change in magnetic field strength produced by movement of the pole piece. In theory, the signal generated by the pole piece moving through the magnetic field creates a representative signal. Many factors adversely affect the “representative signal” that the innovation fails to consider, rendering the design marginal. The velocity with which the pole piece is moved through the sensing coil becomes a critical factor to proper operation. The sensing coil will have different readings due to variations in plunger or pole piece velocities, device orientation, vibration, temperature, contamination, etc. as the plunger or pole piece moves through the sensing coil and encounters the load.
Because many factors affect the velocity with which the plunger or pole piece is moved through the sensing coil, implementation of this device is challenging. Pole piece movement is required to generate a signal (per Faraday's Law of Electromagnetic Induction). This limits the signal output from a sensing coil to a single initial reading. Once the moveable pole piece is in contact with the Target, no further flux change will occur and the sensing coil will no longer produce a voltage difference. If power is cycled or the load shifts, there would be no way to determine the holding force without moving the pole piece back to its original position and restarting the measurement sequence. This means that as a magnet lifts a Target, commonly observed load flexure may introduce an air-gap between the magnet and the Target. Such an air-gap would be undetected, as the measurement period has already transpired. What was presumed to be a safe lift may become an unstable or dangerous lift. While the construction of this device is theoretically possible, this inventor is unaware of any products that have been successfully introduced into the market that utilize the design described by this patent. This inventor does not believe that one skilled in the art can produce a useable working model of the apparatus described by the '455 patent except under ideal conditions.
U.S. Pat. No. 5,096,339 issued to Shoji (1992) describes an electromagnetic drill with Anti-Floating Control Means. The basis of the invention is to provide a sensing method that can detect when an electromagnet base begins to detach from a work Target allowing for the automated shutoff of the drill motor. The basis of the system is to detect magnetic flux levels utilizing a Hall Effect sensor located within the magnetic base in close proximity to the Target. If an air-gap between the machine base and the Target material is introduced during a drilling operation, the magnetic flux level passing through the Hall Effect sensor increases dramatically. When the flux level exceeds a predetermined threshold, integrated electronic circuitry triggers the immediate shutdown of the motor. This technology is limited to simple activation and deactivation of a device. Moreover, placement of the sensor near the Target substantially diminishes the accuracy of the measurement due to localized magnetic field distortion occurring within the Target material.
U.S. Pat. No. 7,201,059 B1 issued to Lin et al. (2007) describes a “Magnetic Force Sensor Assembly For Work-Holding Fixtures.” The goal of the Lin invention is to determine whether sufficient magnetic force exists between a work-holding fixture and a base. This particular design uses a ferromagnetic component, which moves against a spring opposing the magnetic force of the magnet. When the magnetic field of the magnet is of adequate performance it overcomes the spring force and seats the component directly above a position sensor embedded in the work-holding fixture. The spring force can be adjusted to ascertain when a desired grip force has been reached. While the subject sensor assembly can provide the user with the information to determine if adequate grip force exits, the design has considerable difficulty in implementation. The sensor, positioned outside the magnetic assembly, subjects it to accidental impact and environmental damage. Differing magnet orientations have a negative impact on the sensor output. Variations in the spring-constant (due to fatigue, temperature and corrosion) diminish sensor accuracy. Moreover, the use of an excessive number of components increases the likelihood of product failure. The externally located sensor mechanism may interfere with placement of different size materials onto the work-holding fixture. While useful in some machining fixtures, its use in general applications such as below the hook lifting magnets is not recommended. Furthermore, the design does not provide an estimate of the magnetic grip but instead specifies whether and when a particular magnetic attachment force has been attained.
U.S. Pat. No. 8,390,271 B2 issued to Cardone et al. (2013) describes magnetic anchorage equipment with a self-test unit. The basis of the invention is to provide an integrated self-testing unit capable of combining data from multiple data sensors in order to provide an operator with information regarding the ability to lift a desired Target. The purpose of the system is to determine if sufficient magnetic force exists between anchorage equipment and a mold or Target material and ensure that all of the sensors are operational. This particular design uses a characteristic parameter that is proportional to the instantaneous value of the current flowing in one or more polar units identifying the anchorage surface. This unit performs its analysis the moment magnetic equipment is activated. Using magnetic flux signals originating in the coils and mathematically integrating them, a value proportional to the outgoing magnetic attachment force can be derived. Much like the '455 patent discussed above, the magnetic force level is determined only at the instant of activation. This system, in contrast to the '455 patent, does not have moving parts. Combination with the use of numerous self-testing sensors improves product performance. While this design is well suited for lifting large inflexible Targets, such as an injection mold (its intended application), it will have severe limitations when lifting Targets that flex, bend or tilt during operation as the activation process only occurs when the material is stationary and has not yet been lifted.
Thus there remains a need for a sensing method that can quantitatively determine the actual attractive force between a magnet and Target.